Semiconductor structure with airgap

ABSTRACT

A field effect transistor (FET) with an underlying airgap and methods of manufacture are disclosed. The method includes forming an amorphous layer at a predetermined depth of a substrate. The method further includes forming an airgap in the substrate under the amorphous layer. The method further includes forming a completely isolated transistor in an active region of the substrate, above the amorphous layer and the airgap.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to a field effect transistor (FET) with an underlyingairgap and methods of manufacture.

BACKGROUND

RF switches are significantly easier to make on silicon on insulator(SOI) substrates than on bulk substrates because all junctions arebounded by oxide (STI laterally and the buried oxide below) whicheliminates the problem of dropping large voltages across well tosubstrate junctions. SOI also has low junction capacitances whichreduces loading on RF signals. However, it is often advantageous tointegrate an RF switch into a bulk process. This can be done with atriple well and very high resistivity substrates, but is a challenge asthe RF voltages still must be dropped across a junction, and the largedepletion layers in high resistivity substrates add substantial area tothe layout.

SUMMARY

In an aspect of the invention, a method comprises forming an amorphouslayer at a predetermined depth of a substrate. The method furthercomprises forming an airgap in the substrate under the amorphous layer.The method further comprises forming a completely isolated transistor inan active region of the substrate, above the amorphous layer and theairgap.

In an aspect of the invention, a method comprises forming at least onedeep trench structure in a bulk substrate, on sides of an active region.The method further comprises forming sidewall structures on sidewalls ofthe at least one deep trench structure, which acts as an etch stoplayer. The method further comprises forming a lateral undercut in thebulk substrate starting at a bottom of the at least one deep trenchstructure. The method further comprises filling the at least one deeptrench structure with material to form an airgap from the lateralundercut in the bulk substrate under the active region.

In an aspect of the invention, a structure comprises: an amorphous layerunder an active region of a substrate; an airgap in the substrate underthe amorphous layer; and a completely isolated transistor in the activeregion, above the amorphous layer and the airgap and surrounded byshallow trench isolation regions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIGS. 1-4 show respective structures and fabrication processes accordingto an aspect of the present invention; and

FIGS. 5-12 show respective structures and fabrication processesaccording to additional aspects of the present invention.

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to a field effect transistor (FET) with an underlyingairgap and methods of manufacture. In more specific embodiments, thepresent invention is directed to an RF switch FET manufactured in bulktechnology with an airgap underneath its transistor channel. Inembodiments, the present invention provides a completely isolated, e.g.,oxide isolated, switch FET integrated onto the bulk process.

In embodiments, the FET is a bulk CMOS transistor with an underlyingairgap. The location of the airgap, e.g., top of the airgap, isdetermined by an etch barrier directly under and in contact with thetransistor channel and source and drain regions. In embodiments, theetch barrier layer is an amorphous layer formed by an ion implantationprocess. In further embodiments, the location of the sides and/or thebottom of the airgap can be determined by the etch barrier layer. Theairgap, on the other hand, can be formed by NH₄OH wet etch of silicon,where the etch access to the silicon is from a top surface opening. Inalternative embodiments, the etch access to the silicon is from thebottom of a deep trench which has sidewall spacers, and the airgap isformed by XeF₂ dry etch of silicon.

Advantageously, the structures of the present invention fully isolatethe FET so that there is no junction which connects the transistor tothe substrate. The FET of the present invention is also integrated intostandard bulk silicon processing without disturbing adjacent elements.Additionally, the present invention eliminates the problem of droppinglarge voltages across well to substrate junctions in bulk technologies,as well as the problem of large depletion layers in high resistivitysubstrates which add substantial area to the layout.

The FET of the present invention can be manufactured in a number of waysusing a number of different tools. In general, though, the methodologiesand tools are used to form structures with dimensions in the micrometerand nanometer scale. The methodologies, i.e., technologies, employed tomanufacture the level translator of the present invention have beenadopted from integrated circuit (IC) technology. For example, thestructures of the present invention are built on wafers and are realizedin films of material patterned by photolithographic processes on the topof a wafer. In particular, the fabrication of the level translator ofthe present invention uses basic building blocks, including: (i)deposition of thin films of material on a substrate, (ii) applying apatterned mask on top of the films by photolithographic imaging, and(iii) etching the films selectively to the mask.

FIGS. 1-4 show respective structures and fabrication processes accordingto aspects of the present invention. More specifically, in FIG. 1, thestructure 10 includes a BULK substrate 10. In embodiments, the BULKsubstrate 12 is a silicon substrate which can be approximately 350microns in thickness; although other dimensions are also contemplated bythe present invention. A barrier layer 14 is formed on the substrate 12.In embodiments, the barrier layer 14 can be a Silicon Nitride material,which is deposited using conventional deposition processes, e.g.,chemical vapor deposition (CVD) process. Shallow trench isolation (STI)structures 16 are formed in the substrate 12, through the barrier layer14.

In embodiments, the STI structures 16 can be formed from oxide, andfabricated using conventional photolithography, etching, deposition andpolishing processes. For example, in embodiments, a photoresist can beformed on the barrier layer 14, which is exposed to energy (e.g., light)in order to form a pattern. Through conventional etching processes,e.g., reactive ion etching (RIE), a corresponding pattern (vias) isformed in the substrate 12 and barrier layer 14. The photoresist is thenremoved using conventional processes, e.g., oxygen ashing processes. Anoxide or other insulator material is then deposited within theopening(s) and any residual material is removed from the surface of thebarrier layer 14 using, e.g., a chemical mechanical process (CMP).

Still referring to FIG. 1, a photoresist 18 is formed on the barrierlayer 14 and the STI structures 16. The photoresist 18 is patterned toform an opening 18 a between adjacent STI structure(s) 16′. An ionimplant process (as representatively shown by the arrows in FIG. 1) isthen performed to create an amorphous layer 20 at a certain depth of thesilicon layer 12, abutting the STI structure(s) 16′. The amorphous layer20 is formed below the channel, e.g., active region, of transistorswhich will be formed in later fabrication processes. In embodiments, theimplant process can be Ar or Ge or other species, e.g., Boron, whichwill form an amorphous layer 20 from the single crystalline substrate12, e.g., silicon substrate.

In embodiments, the amorphous layer 20 is bounded by the STIstructure(s) 16′ and has a depth of about 500 Å to about 2000 Å;although other depths are contemplated by the present invention asdetermined by the energy level of the ion implantation process. Itshould be understood by those of skill in the art that the depth of theamorphous layer 20 may be a function of the transistor, e.g., in orderto provide sufficient space for a transistor channel, and based on theenergy level of the ion implantation process. A typical amorphising dosefor Ar or Ge will be about 2×10¹³ to 1×10¹⁵ ions per square cm. On theother hand, the dosage of the ion implantation process will determinethe quality of the amorphous layer 20. Both the dosage and energy levelcan be selected using known look-up tables.

In FIG. 2, a trench 22 a and undercut region 22 b is formed on sides ofthe STI structure(s) 16′ and underneath amorphous layer 20,respectively. In embodiments, the undercut portion 22 b extendslaterally below an active region of the yet to be formed transistor, andis of such a depth as to provide sufficient spacing to form an airgapunder such transistor, e.g., about 0.5 microns to about 10 microns. Asshown in FIG. 2, in an alternative or optional embodiment, an amorphouslayer 20 a can be provided under the undercut region 22 b by performinga second, higher energy ion implantation process that the formation ofthe amorphous layer 20. In embodiments, the amorphous layer 20 a is anoptional structure which can be formed prior to or after the amorphouslayer 20.

To form the trench 22 a and undercut region 22 b, a photoresist 24 isformed on the barrier layer 14 and the STI structures 16 (16′). Thephotoresist 24 is patterned to form an opening 24 a. A reactive ion etchprocess is then performed to remove the silicon nitride later and thesilicon material, thereby forming the trench 22 a and undercut region 22b. In embodiments, the undercut region 22 b is formed under an activeregion, e.g., channel, of a yet to be formed transistor. In embodiments,the wet etch process uses a chemistry which is selective to silicon,e.g., which will not attack the oxide material of the STI structure orthe amorphous layer 20 (or amorphous layer 20 a, in optionalembodiments). For example, the wet etch process can be performed usingNH₄OH. In this way, the amorphous layer 20 and oxide of the STIstructures will act as an etch-stop layer. In addition, the photoresist24 will protect the top portion of the wafer, e.g., nitride layer 14.

In FIG. 3, the photoresist is removed and the structure is subjected toan oxidation process to form a passivated surface 26. In embodiments,the passivated surface 26 is an oxidized surface of the substrate 12,e.g., surface of the undercut region 22 b, and of the opposing amorphouslayer 20.

In preferred embodiments, the passivated surface 26 is formed by agrowth process using an annealing process. For example, the structure ofFIG. 3 can be subjected to a high temperature anneal process at about800° C. to about 900° C. In alternate embodiments, the structure of FIG.3 can be subjected to a rapid thermal anneal process to form thepassivated surface 26.

In FIG. 4, the trench 22 a is closed to form an airgap 30. Inembodiments, the trench is closed by the deposition of a material 28,e.g., polysilicon material. After deposition of the material 28, anyresidual material on the surface of the structure can be removed by aCMP process. A conventional transistor (FET) 32 is then formed over theairgap 30 using conventional deposition, lithography, etching andsource/drain formation (diffusion regions on sides of a channel)processes, already known to those of skill in the art. In this way, thetransistor 32 is completely isolated, e.g., oxide isolated, fromportions (undoped portions) of the substrate by the STI structures andpassivated surface 26, with an underlying airgap. The location of theairgap 30 is directly under and in contact with the transistor channeland source and drain regions (diffusion regions), shown representativelyat reference numeral 34. The diffusion regions 34 are thus electricallyisolated from the silicon substrate 12.

In alternative embodiments, additional implants can be performed atmultiple energies to set a perimeter of amorphous material which willlimit the extent of the undercut etch outside of trenches 22 a.

FIGS. 5-12 show respective structures and fabrication processesaccording to additional aspects of the present invention. In FIG. 5, thestructure 10′ includes a pad oxide layer 50 formed on the substrate 12.In embodiments, the pad oxide layer 50 can have a thickness of about 80Å; although other dimensions are also contemplated by the presentinvention. A pad nitride layer 52 is formed on the pad oxide layer 50,which can have a thickness of about 1700 Å; although other dimensionsare also contemplated by the present invention. An oxide hard mask 54 isformed on the pad nitride layer 52, which can have a thickness of about4500 Å; although other dimensions are also contemplated by the presentinvention. In embodiments, the layers 50, 52, 54 can be other materials,any of which are formed using conventional deposition processes, e.g.,CVD, followed by a planarization process, as appropriate, e.g., CMP, asshould be understood by those of skill in the art.

In FIG. 6, a photoresist 56 is formed on the oxide hard mask 54, whichis patterned by exposure to energy (e.g., light). Opening(s) 58 are thenformed in the layers 52, 54, 56, through the pattern, using conventionaletching processes, e.g., RIE.

Thereafter, with an appropriate chemistry, deep trenches 60 are formedin the substrate 12, as shown in FIG. 7. In embodiments, the deeptrenches 60 can be about 5000 Å to about 25000 Å in depth; althoughother depths are also contemplated by the present invention. Forexample, the depth of the deep trenches 60 are provided deep enough toisolate a channel layer (formed in the substrate) of a transistor.

In the case of using the deep trenches 60, continuing with FIG. 8, theresist is removed and sidewall structure(s) 62 are formed in thetrenches 60. In embodiments, the sidewall structure(s) 62 can be anoxide material. More specifically, in embodiments, the sidewallstructure(s) 62 can be formed by an oxide deposition followed by a TEOS(Tetraethyl Orthosilicate) deposition process. In alternativeembodiments, the sidewall structure(s) 62 can be formed using eDRAMcollar deposition processes. In embodiments, the wall thickness of thesidewall structure(s) 62 is dependent on the dimensions of the deeptrench 60; that is, the wall thickness of the sidewall structure(s) 62should not pinch off the deep trench 60. The oxide process can befollowed by an annealing process.

As shown in FIG. 9, the material of the sidewall structure(s) 62 at thebottom of the deep trench 60 is removed by an etching process. Inembodiments, this etching process will also remove the sidewall materialfrom a top surface of the structure, e.g., layer 54. In embodiments, theetching process is an anisotropic etching process, in order to removethe material on horizontal surfaces, leaving the sidewalls on thevertical portions on the deep trench(es).

In FIG. 10, an etching or venting process is performed to form a lateralundercut 64, removing material under an active region, e.g., channel ofa transistor. In embodiments, the etching process is a XeF chemistry.

In FIG. 11, the trench 60 is closed to form an airgap 66. Inembodiments, the trench is closed by the deposition of a material 68,e.g., polysilicon material. After deposition of the material 68, anyresidual material on the surface of the structure can be removed by aCMP process. A conventional transistor (FET) 32 is then formed over theairgap 66 using conventional deposition, lithography, etching andsource/drain formation processes, already known to those of skill in theart. In this way, the transistor 32 is completely isolated.

FIG. 12 shows a top view of FIG. 11. As shown in this view, the filledtrenches, e.g., material 66, surround the transistor 32 such that thetransistor 32 is completely isolated, with an underlying airgap. Thelocation of the airgap 30 is directly under and in contact with thetransistor channel and source and drain regions.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A method, comprising: forming a lateral undercutin a bulk substrate starting at a bottom of at least one deep trenchstructure; filling the at least one deep trench structure with materialto form an airgap from the lateral undercut in the bulk substrate underthe active region; and forming a transistor over the airgap, wherein thelateral undercut is formed by etching with XeF₂ and the material isformed directly on sidewall structures on sidewalls of the at least onedeep trench structure, and wherein the transistor is formed as acompletely isolated transistor.
 2. The method of claim 1, furthercomprising completely filling of the at least one deep trench structurewith the material to form the airgap from the lateral undercut in thebulk substrate under the active region.
 3. The method of claim 2,wherein the sidewall structures comprise depositing oxide within the atleast one deep trench structure and removing of the oxide from a bottomsurface of the at least one deep trench structure to expose the bulksubstrate for subsequent etching.
 4. The method of claim 3, wherein thesidewall structures are formed prior to forming the lateral undercut inthe bulk substrate.
 5. The method of claim 3, further comprisingremoving residual material of the sidewall structures from a surface ofa substrate material on the bulk substrate.
 6. The method of claim 1,wherein a location of the airgap is directly under a transistor channel.7. The method of claim 6, wherein the airgap is in contact with thetransistor channel.
 8. The method of claim 1, wherein the material ispolysilicon material deposited within the at least one deep trenchstructure.
 9. The method of claim 1, wherein the sidewall structures areon vertical portions of the at least one deep trench structure.